The combustion of a carbonaceous material such as a solid carbonaceous fuel by reaction with a source of gaseous oxygen is well known. In such a reaction, an amount of air or oxygen equal to or greater than that required for complete combustion is used, and as a result the gaseous effluent contains carbon dioxide with little, if any, carbon monoxide. It is also known to carry out the gasification or partial oxidation of solid carbonaceous materials or fuels employing a limited quantity of oxygen or air so as to produce primarily carbon monoxide and hydrogen, i.e., synthesis gas.
Various problems are associated with the different types of feeds utilized in gasification processes. With liquid hydrocarbon and petroleum coke feeds there is insufficient ash content in the feed to create a satisfactory slag which is necessary to form a satisfactory insulating layer on the gasifier walls. Nickel and Vanadium build-up also occurs with such feeds if there is not a satisfactory slag flow down the gasifier walls to wash these compounds out of the gasifier. It is taught in. U.S. Pat. No. 4,668,428 that adding iron additives to liquid hydrocarbon and petroleum coke feeds to a gasifier can be beneficial in reducing the viscosity of the slag.
The problem of insufficient ash or nickel/vanadium build-up is not present in coal since coal has from 10-20 percent by weight ash and petroleum coke and heavy liquid hydrocarbons typically only have less than one percent by weight ash. Coal feeds, however, present different problems. Coals are classified according to their rank and BTU content and consequently have different reactivities.
Typically, the higher the BTU content the lower the reactivity of the coal with oxygen. The lower the reactivity the coal has with oxygen, the higher the temperature that is necessary to partially oxidize the coal into synthesis gas. The rating of coals and coke from the lowest BTU content and highest reactivity to the highest BTU content and lowest reactivity is lignite coals, sub-bituminous coals, bituminous coals, anthracite coals, and petroleum coke.
Since bituminous and anthracite coals and petroleum coke are at the top of the scale, they have the least reactivity and require the highest temperatures to obtain adequate conversion into synthesis gas. The high temperatures necessary for the gasification of anthracite or bituminous coal or petroleum coke reduces the efficiency of converting coal to synthesis gas. The high temperatures also shorten the life of the internal components of the gasifier reactor.
It would be advantageous to have a practical and efficient method of gasifying coal within an appropriately moderate temperature range while still avoiding the formation of undesirable compounds. Moderate to high temperatures are necessary to avoid formation of undesirable compounds such as tars, phenols and other aromatics. These compounds pose environmental and safety hazards if emitted into the atmosphere.